Android Kotlin Architecture

Android Kotlin — Architecture Module

Parent kit: ../../SKILL.md · Index: ../../AGENTS.md

Read First

File	When
../../references/01-architecture.md	Full MVVM/MVI patterns, container/content composables
../../references/04-coroutines-flow.md	StateFlow, Channel effects, stateIn
../../references/05-hilt-di.md	@HiltViewModel, repository wiring
../../references/06-room-db.md	Repository + Room offline-first

MVI Contract (Mandatory)

@Immutable
data class FeatureUiState(
    val items: List<ItemUi> = emptyList(),
    val isLoading: Boolean = false,
    val error: UiError? = null
)

sealed interface FeatureUiEvent {
    data class SearchChanged(val query: String) : FeatureUiEvent
    data object Refresh : FeatureUiEvent
    data class ItemClicked(val id: String) : FeatureUiEvent
}

sealed interface FeatureUiEffect {
    data class ShowMessage(@StringRes val messageRes: Int) : FeatureUiEffect
    data class NavigateToDetail(val id: String) : FeatureUiEffect
}

class FeatureViewModel @Inject constructor(
    private val repository: FeatureRepository
) : ViewModel() {

    private val _state = MutableStateFlow(FeatureUiState())
    val state: StateFlow<FeatureUiState> = _state.asStateFlow()

    private val _effects = Channel<FeatureUiEffect>(Channel.BUFFERED)
    val effects: Flow<FeatureUiEffect> = _effects.receiveAsFlow()

    fun onEvent(event: FeatureUiEvent) {
        when (event) {
            is FeatureUiEvent.SearchChanged -> onSearchChanged(event.query)
            FeatureUiEvent.Refresh -> refresh()
            is FeatureUiEvent.ItemClicked -> emitNavigate(event.id)
        }
    }

    private fun onSearchChanged(query: String) {
        _state.update { it.copy(searchQuery = query) }  // ONLY mutation style allowed
    }

    private fun refresh() {
        viewModelScope.launch {
            _state.update { it.copy(isLoading = true, error = null) }
            repository.sync()
                .onSuccess { _state.update { it.copy(isLoading = false) } }
                .onFailure { e ->
                    _state.update { it.copy(isLoading = false, error = e.toUiError()) }
                }
        }
    }

    private fun emitNavigate(id: String) {
        viewModelScope.launch { _effects.send(FeatureUiEffect.NavigateToDetail(id)) }
    }
}

Module Layout (Large Apps)

app/
feature-<name>/
  presentation/   # Composables + ViewModel
  domain/         # Models, repository interfaces, UseCases
  data/           # Room, Ktor, mappers
core/
  core-ui/        # Design system
  core-domain/
  core-testing/

Feature modules must not depend on other feature modules — only core-* and domain contracts.

UseCase Rule

Use UseCase	Skip UseCase
Logic shared by 2+ ViewModels	Single-line repo delegate
Multi-repo orchestration	One ViewModel, one repo
Needs isolated unit tests	CRUD with no extra rules

Anti-Patterns (Architecture)

Banned	Fix
_state.value = newState	_state.update { it.copy(...) }
God ViewModel (>200 lines)	Split screen or extract UseCases
ViewModel in composable params	Pass onEvent lambdas
Domain model with @Entity	Entity in data/, map in repository
Collecting Flow in init without stateIn	stateIn(WhileSubscribed(5_000))